N-Alkyl-substituted aminophenols are of extreme importance in industry as intermediates for heat-sensitive or pressure-sensitive dyes, xanthene dyes, fluorescent dyes, etc.
It is conventionally known to prepare an N-alkyl-substituted aminophenol by reductive alkylation of an aminophenol, which is carried out by continuously feeding an aldehyde or a ketone to a reaction system containing an aminophenol, an organic solvent, a catalyst for reduction, and hydrogen.
However, the conventional processes have the following disadvantages.
Catalysts for reduction generally used in reductive alkylation are noble metal catalysts, e.g., platinum metal catalysts and palladium metal catalysts. These catalysts, when used on an industrial scale, should be used repeatedly because of their expensiveness. However, where a catalyst recovered by filtration of the reductive alkylation reaction mixture is reused in the next reductive alkylation reaction, the catalyst exhibits seriously reduced performance, causing various troubles for use in industry. That is, the rate of reaction is low, which reduces productivity; the main reaction is suppressed and, instead, the aminophenol and the aldehyde or ketone are condensed to form heavy matter; and the aldehyde or ketone is reduced to increase by-production of an alcohol.
For the purpose of preventing reduction in reaction efficiency due to reduced performance of the catalyst, various proposals have hitherto been made.
For example, JP-A-55-100344 discloses a process for exchanging a half of the recovered catalyst with a fresh catalyst (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). This process, however, still requires a large quantity of a fresh catalyst to be added and, hence, is not deemed to establish substantial repeated use of a catalyst for reduction.
JP-A-57-81444 discloses a process in which the amount of an aldehyde to be fed to a reaction system is adjusted to a stoichiometric amount. This process, however, is not always regarded industrially advantageous because, when applied to the production of N-alkyl-substituted aminophenols, too a low amount of the aldehyde fails to make a conversion of the aminophenol, an expensive raw material, nearly 100%. Further, since the rate of reaction becomes low in the latter half of the reaction, the reaction time should be lengthened accordingly.
Further, JP-A-55-20773 discloses a process for preparing N,N-dimethylaminobenzoic acid, in which a spent catalyst for reduction which is recovered from the reaction mixture by filtration is washed with methanol for reuse. This technique, however, is unsuitable for practical use because, as mentioned in the working examples, the reaction time becomes longer each time the recovered and methanol-washed catalyst is reused. Where the process is applied to the preparation of N-alkyl-substituted aminophenols, the reaction system suffers from an increase of a condensation reaction between an aminophenol and an aldehyde or a ketone to form heavy matter, or of a side reaction of an aldehyde or ketone to form an alcohol, thus resulting in remarkable reduction in yield of the desired N-alkyl-substituted aminophenol.
In the light of the above, these conventional techniques are not yet satisfactory from the industrial viewpoint.
It has also been suggested to add a small amount of acetic acid to the reaction system to produce an effect of keeping the catalyst surface clean against contamination as described in JP-A-55-20773. In this process, however, the whole amount of acetic acid is added before commencement of the reaction as described in the working examples. If such a manner is applied to the reaction of aminophenols, a Schiff base of an aminophenol which is very labile and ready to be condensed to form heavy matter undergoes further condensation by the action of the acetic acid added in the initial stage of the reaction, failing to improve the yield of the desired product. This phenomenon is particularly conspicuous when the catalyst is repeatedly used.
JP-A-61-100551 teaches of conduct the reaction under acidic conditions. According to the working examples thereof, since hydrogen in the nascent state is employed, a large quantity of, e.g., hydrochloric acid or acetic acid, should be used, or the reaction with hydrogen in the presence of a catalyst for reduction should be carried out in an acetic acid solvent. If this technique is adopted to the reaction of aminophenols, there is observed reduction in yield due to condensation of a Schiff base of an aminophenol to form heavy matter, and the yield of the desired product attained is very low. Besides, from the industrial standpoint, use of a large quantity of an acid gives rise to a problem of corrosion of equipment.
JP-A-51-19732 discloses a process comprising forming a Schiff base in the presence of a trace amount of an organic carboxylic acid, followed by hydrogenation. This process cannot be applied to the reaction of aminophenols because the Schiff base of aminophenol is very labile.